While the insulating coating of the magnetic steel sheet for use in motors and transformers and the like is required to have a certain interlayer resistance, not limited to that, but various other properties are required in view of convenience in processing and storage. Since application of the magnetic steel sheet is versatile, various types of insulating coating have been developed depending on the application.
The insulating coating can be classified broadly into three types: (1) an inorganic coating sustainable for stress relief annealing, in greater accounts of weldability and heat resistance, (2) a semi-organic coating which consists of inorganic with some organic materials, sustainable for the stress relief annealing, with the aim of combining the punching quality and weldability, and (3) a special-purpose organic coating unsustainable for the stress relief annealing. Regarding the punching quality (that is, a capability of reducing abrasion of the punching die), typically, a tendency of (3), (2), (1) in order is regarded according to better quality.
Recently, with improvement of performance of motors and transformers, performance of the magnetic steel sheet tends to improve accordingly. Since a magnetic steel sheet having excellent magnetic properties has an increased content of Si and the like, there has been a problem that the punching quality becomes bad because of increased hardness of the steel sheet.
A magnetic steel sheet having a low hardness is also required to have a more excellent punching quality to achieve cost reduction by decreasing number of times of die-grinding.
As the semi-organic coating in the above (2), a coating having chromate as a matrix of the coating and resin, such as acrylic resin, epoxy resin, polyvinylacetate, and the like, added to the matrix is mainly used because of comparatively good material characteristics (insulation performance, adhesiveness of coating, corrosion resistance, and the like). Then, as a method for improving the punching quality of the semi-organic coating, use of fluorocarbon-based resin as the resin added to the chromate is proposed.
For example, Japanese Patent Publication No. 4-43715/1992 discloses a method for forming the insulating coating, in which a fluorocarbon resin or polyethylene is dispersed in the chromate (solution), and then the fluorocarbon resin is baked to be concentrated on the surface. In addition, Japanese Patent Publication No. 7-35584/1995discloses a method for forming the insulating coating, in which a treatment liquid, where a phenolic resin, perfluoroalkyloxyethyleneethanol, and the like are dispersed in the chromate, is coated and then baked. Further, Japanese Patent Laid-Open No. 7-278834/1995, Japanese Patent Laid-Open No. 7-286283/1995, and Japanese Patent Laid-Open No. 7-331453/1995 also disclose the magnetic steel sheet in which the fluorocarbon-based resin is present as the outer or inner layer of the resin particles in the inorganic coating consisting of chromate.
Although these methods improve the punching quality of the magnetic steel sheet, the improvement was not sufficient for the magnetic steel sheet having a high hardness. Even when these methods are used for the magnetic steel sheet having a low hardness, still higher punching quality has been also required.
In addition, in these methods, the fluorocarbon resin was unstable and thus uniform and well-adhesive coating was not obtained.
On the other hand, in case the organic coating like (3) was coated, the improvement of the punching quality was also inadequate.
The punched, magnetic steel sheets are stacked and used for cores. In such stacking, the steel sheets must be slid among them to align edges of the stacked steel sheets, however, a poor sliding performance prevents the processing. On the contrary, it is known that the sliding performance is improved by making the surface of the steel sheets dull-like (pear-surface-like) or giving an insulating coating having a rough surface so that air easily enters the space between the steel sheets to reduce sticking of the steel sheets.
On the other hand, with improvement of the performance of the magnetic steel sheet, the thickness of the sheets tends to reduce and number of sheets stacked increases accordingly, thus the sliding performance of the steel sheets is important in the stacking. In this case, for the traditional dull steel sheets, performance of the magnetic steel sheet becomes bad in view of its magnetic properties, and for the steel sheets with the insulating coating having a roughed surface-roughness, there has been a problem that while the stick among the steel sheets reduces, dust is liable to generate.